Page 39 - Valve Selection Handbook

26 Valve Selection Handbook

controlled by leather or plastic back-up rings, which fit tightly around the
valve stem. Manufacturers of O-ring packings supply tables, which relate
the Durometer hardness and the clearance gap around the stem to the fluid
pressure at which the packing is safe against extrusion.
Thrust Packings

Thrust packings consist of a packing ring or washer mounted between
shoulders provided on bonnet and valve stem, whereby the valve stem is
free to move in an axial direction against the packing ring. The initial
stem seal may be provided either by a supplementary radial packing such
as a compression packing, or by a spring that forces the shoulder of the
stem against the thrust packing. The fluid pressure then forces the shoul-
der of the stem into more intimate contact with the packing.
Thrust packings are found frequently in ball valves such as those
shown in Figures 3-61 through 3-63, 3-65, and 3-67.
Diaphragm Valve Stem Seals

Diaphragm valve stem seals represent flexible pressure-containing
valve covers, which link the valve stem with the closure member. Such
seals prevent any leakage past the stem to the atmosphere, except in the
case of a fracture of the diaphragm. The shape of the diaphragm may rep-
16
resent a dome, as in the valve shown in Figure 3-7, or a bellows, as in
the valves shown in Figures 3-6 and 3-39. Depending on the application
of the valve, the construction material of the diaphragm may be stainless
steel, a plastic, or an elastomer.
Dome-shaped diaphragms offer a large uncompensated area to the
fluid pressure, so the valve stem has to overcome a correspondingly high
fluid load. This restricts the use of dome-shaped diaphragms to smaller
valves, depending on the fluid pressure. Also, because the possible
deflection of dome-shaped diaphragms is limited, such diaphragms are
suitable only for short lift valves.
Bellows-shaped diaphragms, on the other hand, offer only a small
uncompensated area to the fluid pressure, and therefore transmit a corre-
spondingly lower fluid load to the valve stem. This permits bellows-
shaped diaphragms to be used in larger valves. In addition, bellows-
shaped diaphragms may be adapted to any valve lift.
To prevent any gross leakage to the atmosphere from a fracture of the
diaphragm, valves with diaphragm valve stem seals are frequently pro-
vided with a secondary valve stem seal such as a compression packing.